This invention relates to an adsorption process for the fractionation or purification of a gaseous mixture. More particularly, the invention relates to an adsorption process for the production of a product gas of substantially constant content of the desired gaseous component.
In addition to the frequently employed adsorption processes wherein the separation of the components of gaseous feed mixtures is based on different equilibrium points of the adsorbents with respect to the individual gaseous components, there are also known adsorption processes, whose operation is based on the varying adsorption rates of the gaseous components on the adsorbents. The latter processes are used to separate gaseous mixtures if the equilibrium points of their gaseous components on the adsorbent employed do not permit separation, because the adsorbent attains equilibrium at substantially different rates for each component of the feed. Fractionation of air by adsorption of O.sub.2 on carbon molecular sieves, which exhibit, at the beginning of the adsorption, substantially different adsorption rates for nitrogen and oxygen is one example of a gas which is fractionated in a process based on different adsorption rates. Such an air fractionation process is disclosed, for example, in DOS (German Unexamined Laid-Open Application) No. 2,702,785, whose entire contents are incorporated herein by reference.
In the operation of such an adsorption process it is frequently difficult to adjust the amount of a product gas produced to a fluctuating demand without appreciably varying the content of the adsorbed component in the product gas. In general, such adsorption plants, operated cyclically, for example by the pressure swing adsorption method, are optimized, to deliver a substantially constant amount of a product gas having constant purity. In the operation of the adsorption processes utilizing the kinetic effect based on the differing adsorption rates of the feed gas components it was heretofore thought that the length of time of adsorption cycles cannot exceed certain time limits and that the rates of flow of feed gases must have certain minimum values, because otherwise, the adsorption due to the kinetic effect either would not take place or would occur only to a negligible extent. If it was desired to reduce the output of a product gas, in response to a decreased product gas demand, i.e., if it was desired to operate the adsorber in a partial-load mode, the procedure heretofore has been to temporarily shut down the adsorption plant at appropriate intervals and to collect the excess amount of product gas obtained in intermediate storage tanks. In addition to the initial investment cost of the storage tanks, another disadvantage of this mode of operation resides in that a certain period of time must first elapse with each renewed start-up of the plant until the product gas has attained the desired purity.